Porosity of a mixture of clumps and spheres

-What's the best way to measure the porosity of a packing which is a mixture of clumps and spheres?
I've created the packing with makeCloud and thus we can get the envelop volume of the packing or even with aabbExtrema().
But, then how do I use this volume to get the porosity?

-Is the porosity computation of yade still valid if clumps are composed of overlapping spheres?